z66j I'ariabU Stars 347 



invisible to the naked eye and at other times to be con- 

 spicuous ; a Dutch astronomer, Phocy tides Holwarda ( 1 6 1 8- 

 1651), first clearly recognised its variable character (1639), 

 and Ismail Boulliau or Bullialdus (1605-1694) in 1667 fixed 

 its period at about eleven months, though it was found that 

 its fluctuations were irregular both in amount and in period. 

 Its variations formed the subject of the first paptr published 

 by Herschel in the Philosophical Transactions (1780). An 

 equally remarkable variable star is that known as Algol 

 (or ft Persei), the fluctuations of which were found to be 

 performed with almost absolute regularity. Its variability 

 had been noted by Geminiano Montanari (1632-1687) in 

 1669, but the regularity of its changes was first detected 

 in 1783 by John Goodricke (1764-1786), who was soon 

 able to fix its period at very nearly 2 days 20 hours 49 

 minutes. Algol, when faintest, gives about one-quarter as 

 much light as when brightest, the change from the first 

 state to the second being effected in about ten hours ; 

 whereas Mira varies its light several hundredfold, but 

 accomplishes its changes much more slowly. 



At the beginning of Herschel's career these and three or 

 four others of less interest were the only stars definitely 

 recognised as variable, though a few others were added soon 

 afterwards. Several records also existed of so-called " new ; ' 

 stars, which had suddenly been noticed in places where no 

 star had previously been observed, and which for the most 

 part rapidly became inconspicuous again (cf. chapter n., 42 ; 

 chapter v., 100 ; chapter vii., 138); such stars might 

 evidently be regarded as variable stars, the times of greatest 

 brightness occurring quite irregularly or at long intervals. 

 Moreover various records of the brightness of stars by earlier 

 astronomers left little doubt that a good many must have 

 varied sensibly in brightness. For example, a small star in 

 the Great Bear (close to the middle star of the " tail ") was 

 among the Arabs a noted test of keen sight, but is perfectly 

 visible even in our duller climate to persons with ordinary 

 eyesight ; and Castor, which appeared the brighter of the 

 two Twins to Bayer when he published his Atlas (1603), 

 was in the i8th century (as now) less bright than Pollux. 



Herschel made a good many definite measurements of 

 the amounts of light emitted by stars of various magnitudes, 



